The present invention relates to an apparatus and method for automatically controlling fluid flow through a valve in response to system changes and more particularly to a self-adjusting valve which automatically adjusts the refrigerant flow through an evaporator in a refrigeration system.
Mechanical expansion valves frequently are used in refrigeration systems to meter the liquid refrigerant flow into an evaporator coil. A type of mechanical expansion valve commonly used contains a body having an inlet port and an outlet port and a movable valve, or closure, element placed within the body for opening and closing the valve orifice. A compression spring is placed at one end of the valve element to apply a force to it in a first direction. A second force generator, such as a fluid-filled bellows or diaphragm, is placed against the other end of the closure element to provide a force against it representative of a system parameter, such as the outlet temperature of the evaporator coil, in a second direction that is opposite to the direction of the force provided by the compressed spring. The second force generator is coupled in fluid communication with a thermal probe, with the second force generator, the probe and the communication conduit therebetween being filled with an appropriate fluid. The probe typically is attached thermally to the evaporator coil outlet. As the temperature of the probe changes, the pressure in the probe, and thus in the second force generator, changes, causing the bellows or diaphragm, for example, to expand or to contract, thereby providing a force at the closure device that is representative of the temperature of the region where the probe is located, such as at the evaporator coil outlet. Examples of various valves may be found in U.S. Pat. Nos. 4,651,535; 5,026,022; 5,065,595; 5,148,684; 5,232,015 and 5,238,219.
Such expansion valves, however, do not have a single set point and, therefore, must be adjusted in response to a change in the operating conditions of the refrigeration system. Such adjustments can be both time-consuming and may necessitate disassembling the system to obtain access to the valve. In addition, if the valve is not adjusted, operating problems and/or power inefficiencies in the refrigeration system operation will typically be the end result. Because, as a practical matter, valves cannot be adjusted continuously in response to every change affecting the operation of the refrigeration system, refrigeration systems have inherent power inefficiencies. Thermomechanical expansion valves which permit remote adjustments to the valve stem have been developed to alleviate the necessity of disassembling the system for adjustment. These devices, however, are susceptible to undesirable leakage and require continued readjustment.
Other attempts have been made to solve some of the aforementioned problems and to increase the power efficiencies of these systems by making the inlet and outlet ports balanced, thus making the opening forces of the valve orifice independent of the pressure drop across the orifice. The problem with these mechanisms is that they fail to compensate for variable flow through the orifice. The flow through an orifice varies as the square root of the pressure drop across the orifice.
The tonnage of a refrigeration system equates to the amount of refrigeration that the system will achieve, i.e. the number of BTUs that it will remove from the refrigerated area. One method of increasing the tonnage of a refrigeration system is to increase the load, i.e. the volumetric flow of the refrigerant through the system. Flow may be increased by increasing the liquid line pressure upstream of the evaporator. A conventional expansion valve is sized for a particular load or refrigerant flow through the system because it is dependent on the liquid line pressure and thus has an orifice sized for a predetermined flow through the valve. The larger the load, the larger the valve orifice. Therefore, a conventional expansion valve has only a limited range of permitted flow through the valve. For example, if the flow is doubled, another expansion valve with a larger orifice must be used because a conventional valve will not operate over a wide range of volumetric flow rates. Further, this deficiency requires that fluctuating liquid line pressures be avoided.
Since conventional expansion valves are dependent on the liquid line pressure, attempts are made to maintain a constant liquid line pressure. This requires that the liquid line pressure be maintained at the high summer pressures even during the colder winter months. Operating the refrigeration system at artificially high pressures during the winter wastes energy and adds expense to the operation of the system. Running at higher discharge pressures also results in higher compressor operating temperatures which decreases the life expectancy of the compressor.
The present invention overcomes these deficiencies of the prior art.